VulnerabilityVector extracted.
Changed error messages. Less arguments to ctor.
Parsing in constructor.
Parsing in constructor, change in error message.
REfined error checking and error messages.
Rerasie exceptions rather than exit program in VulnerabilityVector.

This also implies a change in the main program logic so that excpetions
generate proper termination at the right level when they are caught.
Added separate doctests.
Merge branch 'feature/refactor_cvss' into develop